Circuit breakers of the electromagnetic or thermal type are well-known. Such circuit breakers have trip times in the range of 10 milliseconds to more than one minute, depending on the degree of overload. These trip times may be dangerously too long in many critical circuit breaker applications, such as military, flammable or explosive environments. Furthermore, in large electrical systems, a tripped circuit breaker must be located and subsequently reset manually in order to restore power to the circuit after the fault has been cleared. Other problems associated with such circuit breakers include difficulties associated with reliability, short life, and testing.
The drawbacks associated with electromagnetic and thermal circuit breakers have led to the development of solid-state power controllers (SSPC). SSPC's provide electronic control of the time-to-trip characteristic, e.g., the period from detection of a high load current fault condition to the interruption of power to a load, as opposed to the thermal/mechanical control provided by traditional circuit breakers. This electronic control permits the SSPC to operate much faster for any given overload current, and allows remote operation, e.g., on/off and reset control, of the SSPC. The increased trip speed means that less electrical energy is delivered into an electrical short circuit or overload, thus decreasing the hazard of smoke or flame, and reducing the damage to equipment and electrical wiring. In addition to increased trip speed, the chief advantages of solid-state power controllers over conventional circuit breakers are their improved reliability and life, the ability to control them remotely as a relay substitute, and their built-in test capabilities.
Solid-state power controllers have been developed for numerous applications. U.S. Pat. No. 4,709,160 to Kinoshita discloses a solid-state power controller for DC service. The controller provides a solid-state electrical switch for use in aircraft applications, and is capable of detecting overload conditions, such as those caused by battle damage. It instantly interrupts electrical service to the load, thereby preventing or limiting further damage to the service or load or lead wires. The power controller may be remotely reset after the battle damage or overload condition has been cleared. U.S. Pat. No. 4,955,069 to Ionescu discloses an AC power controller. U.S. Pat. No. 4,740,883 to McCollum discloses a universal solid-state power controller for both AC and DC service.
While shorter trip times are a potential advantage of solid-state power controllers, they can lead to difficulties in the form of nuisance tripping, e.g., undesired turn off of power, when used with certain types of electrical load equipment that draw high starting currents as part of their normal operation. For example, when a lamp is initially energized, the inrush of current may exceed ten times the normal running or operating current. Additionally, for certain motors, the starting surge or current draw may be three to fives times greater than the running current of the motor, and the increased current may last for a second or more. One method for circumventing the high starting current problem is to provide SSPC's with trip times tailored to a particular load's starting current, which would thereby force the manufacturer of equipment having different starting currents to keep a large inventory of SSPC's in stock. In most cases, designers which use SSPC's have chosen to retain the slow trip times of thermal circuit breakers to avoid nuisance trips during load starting, thereby forfeiting the potential advantage of improved smoke and fire protection.